transport layer

Gursharan Singh [email protected]

www.eazynotes.com

Transport Layer

16-May-20111 www.eazynotes.com

IntroductionThe transport layer is the fourth layer from the

bottom in the OSI reference model.

It is responsible for message delivery from process running in source computer to the process running in the destination computer.

Transport layer does not perform any function in the intermediate nodes.

It is active only in the end systems.

16-May-20112 www.eazynotes.com

IntroductionData Link Layer is responsible for delivery of

frames between two neighboring nodes over a link.

This is called node-to-node delivery.

Network Layer is responsible for delivery of datagrams between two hosts.

This is called host-to-host delivery.

Transport Layer is responsible for delivery of entire message from one process running on source to another process running on destination.

This is called process-to process delivery.

16-May-2011www.eazynotes.com3

Transport Layer Design Issues

The transport layer delivers the message from one process to another process running on two different hosts.

Thus, it has to perform number of functions to ensure the accurate delivery of message.

The various functions of transport layer are:

Establishing, Maintaining & Releasing Connection

Addressing

Data Transfer

Flow Control

Error Control

Congestion Control



Establishing, Maintaining & Releasing Connection:

The transport layer establishes, maintains & releases end-to-end transport connection on the request of upper layers.

Establishing a connection involves allocation of buffers for storing user data, synchronizing the sequence numbers of packets etc.

A connection is released at the request of upper layer.



Addressing:

In order to deliver the message from one process to another, an addressing scheme is required.

Several process may be running on a system at a time.

In order to identify the correct process out of the various running processes, transport layer uses an addressing scheme called por number.

Each process has a specific port number.



Data Transfer:

Transport layer breaks user data into smaller units and attaches a transport layer header to each unit forming a TPDU (TransPort Layer Data Unit).

The TPDU is handed over to the network layer for its delivery to destination.

The TPDU header contains port number, sequence number, acknowledgement number, checksum and other fields.



Flow Control:

Like data link layer, transport layer also performs flow control.

However, flow control at transport layer is performed end-to-end rather than node-to-node.

Transport Layer uses a sliding window protocol to perform flow control.



Error Control:

Transport layer also provides end-to-end error control facility.

Transport layer deals with several different types of errors:

Error due to damaged bits.

Error due to non delivery of TPDUs.

Error due to duplicate delivery of TPDUs.

Error due to delivery of TPDU to a wrong destination.



Congestion Control:

Transport layer also handles congestion in the networks.

Several different congestion control algorithms are used to avoid congestion.


Transport Layer ServicesTransport layer protocols can provide two

types of services:

Connection Oriented Service

Connectionless Service


Transport Layer ServicesConnection Oriented Service:

In connection oriented service, a connection is first established between sender and the receiver.

Then, transfer of user data takes place.

At the end, connection is released.

The connection oriented service is generally reliable.

Transport layer protocols that provide connection oriented service are TCP and SCTP (Stream Control Transmission Protocol).


Transport Layer ServicesConnectionless Service:

In the service, the packets are sent from sender to receiver without the establishment of connection.

In such service, packets are not numbered.

The packets may be lost, corrupted, delayed or disordered.

Connectionless service is unreliable.

Transport layer protocol that provides this service is UDP.


Elements of Transport Protocols

Addressing:

In order to deliver data from one process to another, address is required.

In order to deliver data from one node to another, MAC address is required.

Such an address is implemented at Data Link Layer and is called Physical Addressing.



Addressing (Cont.):

In order to deliver data from one network to another, IP address is required.

Such an address is implemented at Network Layer and is called Logical Addressing.

Similarly, in order to deliver data from a process running on source to process running on destination, transport layer defines the Service Point Address or Port Numbers.



Port Numbers:

Each communicating process is assigned a specific port number.

In order to select among multiple processes running on a destination host, a port number is required.

The port numbers are 16-bit integers between 0 and 65,535.



Port Numbers (Cont.):

Port numbers are assigned by Internet Assigned Number Authority (IANA).

IANA has divided the port numbers in three categories:

Well Known Ports: The ports ranging from 0 to 1023. For e.g.: HTTP: 80, SMTP: 25, FTP: 21.

Registered Ports: The ports ranging from 1024 to 49,151. These are not controlled by IANA.

Dynamic Ports: The ports ranging from 49,152 to 65,535. These can be used by any process.



Socket Address:

Socket address is a combination of IP address and port number.

In order to provide communication between two different processes on different networks, both IP address and port number, i.e. socket address is required.



Multiplexing & Demultiplexing:

A network connection can be shared by various applications running on a system.

There may be several running processes that want to send data and only one transport layer connection available, then transport layer protocols may perform multiplexing.

The protocol accepts the messages from different processes having their respective port numbers, and add headers to them.



Multiplexing & Demultiplexing (Cont.):

The transport layer at the receiver end performs demultiplexing to separate the messages for different processes.

After checking for errors, the headers of messages are dropped and each message is handed over to the respective processes based on their port numbers.



Connection Establishment:

Before communicating, the source device must first determine the availability of the other to exchange data.

Path must be found through the network by which the data can be sent.

This is called Connection Establishment.



Connection Establishment (Cont.):

Connection establishment involves Three-Way Handshaking mechanism:

The source sends a connection request packet to the destination.

The destination returns a confirmation packet back to the source.

The source returns a packet acknowledging the confirmation.



Connection Release:

Once all of the data has been transferred, the connection must be released.

It also requires a Three-Way Handshaking mechanism:

The source sends a disconnect request packet to the destination.

The destination returns a confirmation packet back to the source.

The source returns a packet acknowledging the confirmation.


Transport Layer ProtocolsTransport layer provides two types of services:

Connection Oriented Service

Connectionless Service

For this, transport layer defines two different protocols:

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)


Transmission Control Protocol

Transmission Control Protocol (TCP) is a connection oriented protocol that provides reliable services between processes on different hosts.

It uses the services of lower layer which provide connectionless and unreliable service.


Transmission Control Protocol

The basic features of TCP are:

It provides efficient method for numbering different bytes of data.

It provides stream data transfer.

It offers reliability.

It provides efficient flow control.

It provides full duplex operation.

It provides multiplexing.

It provides connection oriented service.


TCP SegmentTCP segment is the unit of data transferred

between two processes.

Each TCP segment consists of two parts:

Header Part

Data Part


Format of TCP Segment


Format of TCP SegmentSource Port:

It indicates the port number of a source process. It is of 2 bytes.

Destination Port:

It indicates the port number of destination process. It is also 2 bytes.

Sequence Number:

It specifies the number assigned to the current message. It is of 4 bytes.


Format of TCP SegmentAcknowledgement Number:

It indicates the sequence number of the next byte of data. It is of 4 bytes.

Header Length:

It indicates number of words in the TCP header. It is a 4 bit field.

Reserved:

This 6 bit field is reserved for future use.


Format of TCP SegmentFlags:

This 6 bit field consists of 6 different flags:

UGR (Urgent Pointer)

ACK (Acknowledgement)

PSH (Request for Push)

RST (Reset the Connection)

SYN (Synchronize)

FIN (Final or Terminate the Connection)


Format of TCP SegmentWindow:

It specifies the size of sender’s receiving window, i.e., the buffer space available for incoming data. It is of 2 bytes.

Checksum:

This 16-bit field contains the checksum.

Urgent Pointer:

This 16-bit field is valid only if urgent pointer in flags is set to 1.


Format of TCP SegmentOptions:

It contains the optional information in the TCP header. It is of 32 bytes.

Data:

This field contains the upper layer information. It is of variable size.


User Datagram ProtocolUser Datagram Protocol (UDP) is a

connectionless, unreliable transport protocol.

Like TCP, UDP also provides process-to-process communication.

Unlike TCP, it does not provide flow control and error control mechanisms.

It is connectionless, therefore, it transfers data without establishing a connection.


User Datagram ProtocolThe various features of UDP are:

It provides connectionless transport service.

It is unreliable.

It does not provide flow control and error control.

It is less complex and is simple than TCP, and easy to implement.

User datagrams (packets) are not numbered.


UDP DatagramA datagram is the unit of data transferred

between two processes.

Each UDP datagram consists of two parts:

Header Part

Data Part.


Format of UDP Datagram


UDP DatagramSource Port:

It indicates the port number of source process. It is of 16 bits.

Destination Port:

This 16 bit field specifies the port number of destination process.


UDP DatagramLength:

It specifies the total length of the user datagram (header + data). It is of 16 bits.

Checksum:

The contains the checksum, and is optional. It is also of 16 bits.


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transport layer

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